1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. More particularly, the present invention relates to a semiconductor device with a thermoelectric conversion function and discloses a technique suitable for a thermal image input device that is used for providing security or in the field of an ITS (Intelligent Transportation System).
2. Description of the Related Art
To conduct efficient heat detection, it is necessary to suppress heat radiation from a heat (infrared rays) detecting portion. For suppressing the heat radiation, a heat insulating layer containing a considerable amount of air is suitable. Conventionally, with the aim of providing such a heat insulating layer under the heat detecting portion, etching a silicon substrate so as to form a heat separation area having a hollow structure in the substrate has been proposed. (see JP 8(1996)-122162 A, for example). For etching the silicon substrate, an anisotropic etchant, e.g., an alkaline etchant such as KOH, hydrazine, or the like, has been used.
However, etching the silicon substrate takes a long time and thus prevents an improvement in productivity. In addition, it brings about a contamination problem and thus is not compatible with a mass production process of devices using a silicon semiconductor. Moreover, when the anisotropic etchant is used for etching, a (100) surface of the silicone is etched faster that a (111) surface of the silicon. Accordingly, when a silicon substrate of (100) surface orientation is used, respective side walls of the hollow portion make an angle of about 54xc2x0 with the main surface, resulting in tapered side walls. To form the heat separation area under the heat detecting portion while providing allowance for such inclination of the side walls, an area larger than the heat detecting portion needs to be etched. An increase in an occupied area per element thus cannot be avoided, which makes a high-density arrangement of heat detecting portions (heat sensors) and on-tip consolidation with a visible light sensor difficult.
Especially in a heat sensor requiring a cold junction and a hot junction, for which a Seebeck type sensor is a typical example, it is desirable to increase a distance between the cold junction and the hot junction in order to improve the sensitivity of the sensor. However, as the distance between the junctions increases, the heat separation area with the tapered side walls occupies a still larger area on the surface of the substrate. Consequently, problems such as high manufacturing cost due to increases in size of an element and in diameter of an optical system become more significant.
Therefore, the present invention provides a semiconductor device allowing a high-density arrangement of heat detecting portions. The present invention also provides a method of manufacturing a semiconductor device allowing a high-density arrangement of heat detecting portions, which is highly compatible with a mass production process of devices using a silicon semiconductor.
In order to achieve the above, a semiconductor device according to the present invention includes a silicon substrate; a heat insulating layer including a silicon oxide film, which is formed on the silicon substrate; and a heat detecting portion formed on the heat insulating layer. The heat insulating layer includes a closed cavity and/or a hole, an interior of the hole has a greater diameter than an opening of the hole, and at least a portion of the closed cavity or the hole is formed within the silicon oxide film.
When used herein, the term xe2x80x9cclosed cavityxe2x80x9d means a closed space that has no opening extending to the surface of the heat insulating layer and thus is isolated from outside air above the surface of the heat insulating layer. On the other hand, the term xe2x80x9cholexe2x80x9d means a recess with an opening extending to the surface of the heat insulating layer.
In the above-mentioned semiconductor device, the heat insulating layer including the closed cavity and/or the hole exhibits excellent heat insulating properties. Further, the above-mentioned hole is advantageous in achieving high-density arrangement of heat detecting portions because the hole is widened in its interior but not in its opening positioned in the vicinity of the surface of the heat insulating layer. Further, forming the closed cavity is still more advantageous in achieving the high-density arrangement of heat detecting portions because it allows the utilization of the entire surface of the heat insulating layer.
Further, a method of manufacturing a semiconductor device according to the present invention includes the acts of: forming a silicon oxide film on a silicon substrate; forming a silicon polycrystalline film on the silicon oxide film as at least a portion of a heat insulating layer; forming a hole extending through the silicon oxide film and a silicon polycrystalline film by dry etching, the hole having an opening and an interior; oxidizing at least a portion of the silicon polycrystalline film that is in contact with an opening of the hole so that the opening is closed or a diameter of the opening is made smaller than that of an interior of the hole; and forming a heat detecting portion on a heat insulating layer.
According to this manufacturing method, the above-mentioned semiconductor device can be manufactured only by acts that are highly compatible with a so-called silicon mass production process, without using an alkaline etchant such as KOH, hydrazine, or the like.